i know that B = μH, where B is the magnetic flux density, H is magnetic field density and μ is the permeability of the medium. however, i'm wondering what's the difference between magnetic flux density,B and magnectic field density,H? besides, i have the same problem about an electrostatic equation, which is D = εE , where D is electric flux density, E is electric field density and ε is permitivity of the medium. why there's still having D while we already have E, since both of the D and E have the same unit. is their difference is that the magnitud of B and D are dependent to the permitivity and permeability of the medium respectively? on the other hand, H and E are independent of the medium?